1. Field of the Invention
The present invention generally relates to wireless communication systems, and more particularly, to handover of video calls from a packet switched network to a circuit switched network in a single radio environment.
2. Description of the Related Art
An SAE/LTE system is a packet switched (PS) system in which voice and video calls are established through a PS domain. Therefore as a “default option”, voice calls use the PS domain as packet video-calls and underlying call control protocol is SIP supported through an IP multimedia subsystem (IMS). Recently, there has been a slow migration of users from the SAE/LTE domain from a circuit switched (CS) domain to the LTE packet switched domain. Hence, the LTE system may be deployed in “islands” overlaying parts of the CS domain. This means that if a user makes a video-call over the LTE system, the video-call may not be just subject to an intra-domain handover (i.e., radio-level and intra-network-level handover) but is also likely to be subject to an inter-domain handover from the LTE packet domain to the CS domain due to mobility of the user during the video-call. This leads to the development of a functionality called Single Radio Voice Call Continuity (SRVCC) in the Third Generation Partnership Project (3GPP) technology. SRVCC is a functionality that allows a voice/video call in the LTE packet domain to be moved to the CS domain.
CS video-calls support a feature called “Service change for UDI/RDI fallback” (normally referred to as a service change and unrestricted digital information fallback (SCUDIF)). This service is available to UDI/RDI multimedia calls and allows users to achieve successful call establishment when end-to-end CS multimedia is not possible (fallback to speech) or when signaling of the feature is not possible in the CS network (fallback to preferred service or speech). Furthermore, it allows users to swap between a multimedia service and basic speech during an established call.
Nevertheless, in the case that the call (either voice-call or video-call) has been established in the IMS, there is no negotiation that normally takes place between user equipment (UE) and a mobile service centre (MSC) in order to mutually identify whether both the UE and MSC support the SCUDIF feature; hence it is not possible to use the SCUDIF feature when the UE moves to the CS domain from the PS domain. For example, if a voice/video call has been initiated with the PS network and later handed over to the CS network using an SRVCC, there is no communication that takes place between the UE and the MSC during the call setup time. As a consequence, the UE and the MSC are unaware of each other's SCUDIF capabilities (user-initiated or network-initiated) when the voice/video call is transferred to the CS network.
Further, a delay may be introduced by H.324 when performing the following steps for call setup:                H.223 Multiplexer level detection        Terminal Capability Exchange        Master Slave determination        Open Logical Channels        Multiplexer Table Entries Exchange        
The procedure typically takes 5˜8 seconds. If link quality deteriorates or media configurations between UEs are not well-matched, this delay may increase even further. The delay can be suppressed to as low as a few seconds in limited cases when the acceleration techniques are supported by both UEs and little data is lost during the period. However, the call set up procedure of 3G-324M, outlined above, is likely to occur at cell edges under the SRVCC situations where radio link is unstable.
Given that the period of the current SRVCC handover with voice only is significantly smaller (e.g. in the range of 300-500 ms) if simultaneous transfer of voice and video is performed when a handover from PS-to-CS network with video SRVCC (given that a 64 kbps bearer is established on the UTRAN side and the increased call setup delay of 3G-324M from the negotiation between UEs using H.245 signaling procedures) interruption time might be significantly large, during which a message might be displayed to ask for patience from the user or recently-played video clips might be replayed until newly-decoded scenes become available.